1. Field of the Invention
The present invention relates to an image-forming apparatus of an electrophotographic system, such as a laser printer or a copying machine.
2. Description of the Related Art
In recent years, in an image-forming apparatus of an electrophotographic system, there is a need to shorten the first print out time (FPOT) taken until the first page of a recording material is output after a print command is issued. In order to shorten the FPOT, it is necessary to shorten the time (warm-up time) required for warming up a fixing unit that is mounted on the image-forming apparatus. Although the warm-up time of the fixing unit may be shortened by supplying power that is as large as possible to a heater of the fixing unit in a warm-up period, the heater may be damaged if excessively large power is supplied to the heater. Thus, constant power may be supplied to the heater in the warm-up period. However, the resistance value of the heater generally changes with temperature. Such characteristics are referred to as the temperature coefficient of resistance (TCR). When the resistance value of the heater changes with temperature, the power supplied to the heater changes even if the duty ratio of power supplied to the heater is constant. Thus, when the heater has a positive temperature coefficient of resistance, for example, the power supplied to the heater decreases gradually and the warm-up time increases. Japanese Patent Application Publication No. H8-30125 discloses correcting the duty ratio according to the TCR of a heater so that constant power can be supplied to the heater in a warm-up period. However, in recent image-forming apparatuses, current consumption of an entire apparatus, including peripheral devices, tends to increase due to an increase in the number of print-outs per unit time, an increase in the number of peripheral devices (optional devices) connected to the image-forming apparatus, and the like. In such an image-forming apparatus, which consumes a large amount of current, as described above, when constant power is supplied to the heater, the current consumption of the entire apparatus may exceed the rated current 15 A.